In your tutorial Making Sprites From Models for Use in (G)ZDooM you mention Jed's model viewer and how to take screenshots for various rotations. When I try to do that, the tga file turns out to be completely white. I tried 1.3.5 and 1.3.6 with the same result.

Since that obviously works properly for you, did you do anything specific to take the screenshots?

... the tga file turns out to be completely white.... Since that obviously works properly for you, did you do anything specific to take the screenshots?

I don't recall I needed to do anything specific while taking the screenshots. What is the file size of the .tga images of your screenshots? This may indicate if the relevant data are being captured. If the file size is reasonable (I recall getting 2-3 MB images), then you might try opening the files in an alternative graphics editor. I like XnView because it is versatile and free. However, you can also try GIMP or Wally, both of which are free and versatile.